1. Field of the Invention
The invention relates to a glass-ceramic cooktop having a flat upper side forming a cooktop and a smooth underside, wherein the glass-ceramic material of the cooktop has transmittance values of greater than 0.1% in the visible light range in the total wavelength region greater than 420 nm, a light transmittance in the visible range of 0.8-2.5%, and a transmittance of 0-85% in the infrared at 1600 nm, and wherein the glass-ceramic material has high quartz mixed crystals as the prevalent crystal phase.
2. Description of Related Art
Cooktops having a glass-ceramic plate as a cooking surface are familiar prior art. These glass-ceramic plates are usually present as flat plates or are shaped three-dimensionally.
Glass ceramics with high quartz mixed crystals as the prevalent crystal phase are produced from crystallizable lithium aluminum silicate glasses.
These glass ceramics are produced in several steps.
In the large-scale technical production of glass ceramics, first the crystallizable initial glass made up of a mixture of shards and powder-form batch raw materials is melted at temperatures usually between 1500 and 1650° C. Typically, arsenic and/or antimony oxide is used as a refining agent in the melt. These refining agents are compatible with the required glass-ceramic properties and lead to good bubble qualities of the melt. Although these substances are rigidly bound in the glass framework, they are a disadvantage from the points of view of safety and environmental protection. Thus, special precautionary measures must be taken in the recovery and treatment of raw materials and due to evaporation in the melt.
Recently, the particular use of SnO2 as an unobjectionable refining agent has been described. In order to obtain good bubble qualities, at conventional melting temperatures (a maximum of approximately 1680° C.), in addition to SnO2 preferably halide compounds are used as additional refining agents. Thus, the use of 0.1-2 wt. % SnO2 and 0-1 wt. % Cl is described in the Japanese Patent Applications JP 11 100 229 A and JP 11 100 230 A. According to these publications, coloring by addition of V2O5 as the only colorant has been achieved.
The addition of 0.05-1 wt. % fluorine (US 2007 0004578 A1) and 0.01-1 wt. % bromine (US 2008 0026927 A1) for support of refining with SnO2 is also disclosed. Refining temperatures below 1700° C. have also been described in these publications. The primary colorant is V2O5. The addition of halides is a disadvantage, since they vaporize greatly at the melting temperature and thus form toxic compounds, such as, e.g., HF.
The use of SnO2 in combination with high-temperature refining above 1700° C. in order to obtain good bubble qualities is described in DE 199 39 787 C2. This publication, however, provides no indication for obtaining a good display capability in the wavelength range starting from 420 nm.
After melting and refining, the glass usually undergoes a hot shaping by rollers or more recently floats, in order to produce plates. For an economical production, on the one hand, a low melting temperature and a low processing temperature VA are desired; on the other hand, the glass should not show any devitrification during the shaping. That is, no disruptive crystals that would adversely affect the strength in the initial glasses and the glass ceramics produced therefrom should be formed. Since shaping takes place in the vicinity of the processing temperature VA (viscosity of 104 dPas) of the glass, it must be assured that the upper devitrification temperature of the melt lies in the vicinity of and most favorably below the processing temperature, in order to avoid the formation of disruptive crystals.
Subsequently, the initial glass is converted into the glass-ceramic article by controlled crystallization. This ceramicizing takes place in a two-step temperature process, in which first nuclei are produced by nucleation at a temperature between 680 and 800° C., usually from ZrO2/TiO2 mixed crystals. SnO2 can also participate in the nucleation. With subsequent increase in temperature, the high quartz mixed crystals grow on these nuclei. High rates of crystal growth, such as are desired for an economical, rapid ceramicizing, are obtained at temperatures of 850 to 950° C. For this maximum production temperature, the structure of the glass ceramics is homogenized, and the optical, physical and chemical properties of the glass ceramics are adjusted. If desired, the high quartz mixed crystals can subsequently still be converted into keatite mixed crystals. The transformation into keatite mixed crystals is produced in the case of an increase in temperature in a range from approximately 950 to 1200° C. With the transition from high quartz to keatite mixed crystals, the thermal expansion coefficient of the glass ceramics is increased and the transparency is reduced due to the light scatter that accompanies the enlargement of the crystals. As a rule, glass ceramics with keatite mixed crystals as the principal phase are thus translucent or opaque and the light scatter associated therewith acts negatively on the display capability.
A key property of these glass ceramics with high quartz mixed crystals as the principal crystal phase is the ability to produce materials that provide an extremely low thermal expansion coefficient of <0.5×10−6/K in the range from room temperature up to 700° C. and above. Based on the low thermal expansion, these glass ceramics possess an excellent resistance to differences in temperature and stability relative to fluctuating temperatures.
In the application as a cooktop, the technical development based on requirements from practical use leads to very specific, partially contradictory requirements for transmittance.
In order to prevent a disruptive view onto the technical components below the glass-ceramic cooktop and in order to avoid the dazzling effect due to radiating heating elements, in particular bright halogen heating elements, glass-ceramic cooktops are limited in their light transmittance. For display capability, however, a certain light transmittance is necessary in order to assure sufficient brightness with the use of commercial components, e.g., signal generators, LEDs, etc. In order to satisfy these requirements, glass-ceramic cooktops are usually adjusted to light transmittance values of 0.5 to 2.5%. This is achieved by additions of coloring elements. Glass-ceramic cooktops then appear to be black in a top view, due to the low light transmittance, no matter what the coloring element used, while in a transparent view, they appear for the most part red, red-violet or orange-brown according to the coloring elements used.
Color displays are composed of electronic components emitting light, for the most part light-emitting diodes, which are incorporated below the cooktop. They are particularly necessary in the case of induction cooktops for ease of operation and safe operation. For example, the actual heating power or residual heat of the different cooking zones is optically displayed. The display of the residual heat is important for safe handling when the heating elements are not turned on or, as in the case of inductively heated cooking surfaces in general, it cannot be ascertained that the cooktop is hot. The usual red light-emitting diodes irradiate at wavelengths of around 630 nm. In order to improve ease of operation and technical functions, but also to offer the possibility for household appliance manufacturers to differentiate their designs, in addition to the usual red display, displays of other colors are also desired.
Cooktops of Japanese origin are known, in which an LCD display is present, which can be backlit in green, orange and red.
The most varied colors that are used herewith, with the exception of the color red, at the present time serve exclusively for esthetic purposes. The color red, however, usually always indicates danger.
Safety information is only coded and known via display elements or symbols of the same color in seven-segment displays. In safety-critical situations, the user is forced to think of which display the wants to turn on. Added to this is the fact that there is a flood of information available to the user due to the high degree of technology in kitchens and the many appliances present in kitchens, such as cooking ovens, baking ovens, microwaves, grilling devices, hoods, refrigerating and freezing appliances, as well as bread slicing machines, etc., the information being different from one device to another. For example, a red blinking light in one appliance can indicate a danger, while in another device, it indicates an operation.
In commercial colored cooktops, the user cannot recognize the operating state and error condition by means of colors, thus whether the appliance is ready to operate or whether an indication of a possible error condition is present.
An earlier type of glass-ceramic cooktop, known under the name Ceran Color®, produced by SCHOTT AG, possessed good color display capability. Ceran Color® is colored by additions of NiO, CoO, Fe2O3 and MnO and refined by Sb2O3. A light transmittance of typically 1.2% is adjusted for cooktops with a usual thickness of 4 mm by this combination of color oxides. The transmittance in the range of 380 nm to 500 nm is 0.1-2.8%, depending on wavelength in each case. In the case of a wavelength of 630 nm that is common for red light-emitting diodes, the transmittance amounts to approximately 6%. It is a disadvantage in this earlier type of glass-ceramic cooktop that the color oxides used also absorb very strongly in the infrared. The IR transmittance at 1600 nm amounts to less than 20%. Thus, the rate of cooking is reduced. The transmittance curve of Ceran Color® is illustrated in the book “Low Thermal Expansion Glass Ceramics”, Editor Hans Bach, Springer Publishing Co. Berlin Heidelberg 1995, on page 66 (ISBN 3-540-58598-2). The composition is listed in the book “Glass-Ceramic Technology”, Wolfram Hõland and George Beall, The American Ceramic Society 2002 in Tables 2-7.
In more recent, further developed glass-ceramic cooktops, for the most part V2O5 is used for coloring, since it has the special property of absorbing in the visual light range and permitting a high transmittance in the range of infrared radiation.
The coloring by V2O5 is represented as a very complex process. As was shown in earlier investigations (DE 19939787 C2), a redox process is a prerequisite for converting the vanadium oxide to the coloring state. In crystallizable initial glass, the V2O5 still colors relatively weakly and produces a light green color shade. In the ceramicizing, the redox process occurs, the vanadium is reduced and the redox partner is oxidized. The refining agent functions as the primary redox partner. This was shown by Mõssbauer investigations of Sb and Sn-refined compositions. In the ceramicizing, a part of the Sb3+ or Sn2+ in the initial glass is converted to the higher oxidation state Sb5+ or Sn4+. It can be assumed that the vanadium is incorporated in the seed crystal in the reduced oxidation state as V4+ or V3+ and is intensively colored therein due to electron charge-transfer reactions. Also, as another redox partner, TiO2 can reinforce the coloring by vanadium oxide. In addition to the type and quantity of the redox partners in the initial glass, the redox state that is adjusted in the glass for the melt also has an influence. A lower oxygen partial pressure pO2 (melt adjusted as reducing), e.g., due to high melting temperatures, reinforces the coloring effect of the vanadium oxide.
The ceramicizing conditions have another influence on the coloring effect of the vanadium oxide. In particular, high ceramicizing temperatures and longer ceramicizing times lead to a more intense coloring.
The described relationships for coloring by means of V2O5 will be useful for the person skilled in the art, in order to establish the desired transmittance curve by means of a specific glass composition, specific redox adjustments of the pO2 for the melt and the ceramicizing conditions. Previously, however, it was not possible to achieve all requirements, such as light transmittance and high IR transmittance in compliance with specifications, as well as display capability for standard red light-emitting diodes together with the desired improved display capability for light-emitting displays of other colors.
The form of the absorption bands of the vanadium oxide and thus transmittance in the visible light range in the entire wavelength region greater than 450 nm up to the upper limit of 750 nm could not be adapted to higher transmittances.
Examples of such types of V2O5-colored glass-ceramic cooktops are the Sb2O3-refined Ceran Hightrans® and the SnO2-refined Ceran Suprema®, which are produced by the company SCHOTT AG. The transmittance curves of these two glass ceramics are published in the book “Low Thermal Expansion Glass Ceramics”, Second Edition, Editor Hans Bach, Dieter Krause, Springer Publishing Co. Berlin Heidelberg 2005, on page 63 (ISBN 3-540-24111-6).
The transmittance value of 0.1% is not exceeded in the case of the named glass-ceramic cooktops and for other glass-ceramic cooktops found on the market in the wavelengths of approximately 450-550 nm that are important for the visibility of color displays, in particular blue and green displays. Other essential requirements for transmittance are fulfilled by these glass-ceramic cooktops: high infrared transmittance for high rates of cooking, transmittance in compliance with specifications for standard red light-emitting diodes at approximately 630 nm and a light transmittance of about 1.5%.
In order to eliminate this disadvantage, the European Patent Application EP 1465460 A2 discloses a glass-ceramic cooktop that has a Y value (brightness) of 2.5-15 for a thickness of 3 mm, measured in the CIE color system with standard light C. The designations “brightness” and light transmittance correspond to the same measurement value. The Y value is identical to the value of light transmittance, measured according to DIN 5033. Improved displays for blue and green light-emitting diodes will be obtained with this light transmittance. The disclosed compositions are refined with As2O3 and/or Sb2O3, partially in combination with SnO2. The coloring is carried out by means of V2O5.
It is pointed out in the comparative example that the display capability for blue and green light-emitting diodes having the listed material compositions is insufficient for a light transmittance of 1.9%. The claimed high values of light transmittance of at least 2.5% and preferably higher, however, are disadvantageous with respect to hiding the electronic components underneath the cooktop. In addition, the esthetic black appearance of the cooktop from a top view is adversely affected.
A cooktop of glass-ceramic material is known from DE 10 2009 013 127 A1, which provides transmittance values of greater than 0.1% in the visible light range in the total wavelength region greater than 420 nm, a light transmittance in the visible range of 0.8-5%, (preferably 0.8-2.5%) and a transmittance of 45-85% in the infrared at 1600 nm. With such a cooktop, it is assured that the disruptive transparent view onto the technical components underneath the glass-ceramic cooktop is prevented and the esthetic black appearance in the view from the top remains assured. Radiant heating elements are visible during operation and common red light-emitting diode displays can be well recognized. Due to the transmittance of more than 0.1% in the visible light range in the total wavelength region greater than 450 nm, displays of other colors are also well recognizable. In view of the luminosity of commercial blue, green, yellow or orange light-emitting diodes, this transmittance value is sufficient and represents a clear improvement when compared with the prior art. In particular, displays having blue and green colors are clearly improved. Displays with white light are less falsified in color due to the transmittance curve in the entire wavelength region above 450 nm.
The glass-ceramic plates for the cooktops are shaped by upper and lower rollers via a special rolling process. The melted liquid initial glass is introduced into the rollers via a drawing nozzle. The rollers are composed of a special material in order to assure a controlled heat extraction between glass and rollers. Uncontrolled crystallization in the molds of the glass strip must be avoided during the hot shaping by the rollers. The glass strip is guided over a roller table into an annealing lehr. The glass strip is initially kept at temperatures higher than the transformation temperature and lower than the nucleation and crystallization temperatures of the initial glass, in order to reduce possible stresses. After cooling the glass strip to room temperature, the glass strip is cut, the edges are processed, it is decorated with ceramic colors and subsequently transformed into glass ceramics in the ceramicizing oven.
These glass-ceramic plates for cooktops possess a knob-like underside structure in order to fulfill the strength requirements for glass-ceramic plates for cooktops. These knobs are embossed in the underside of the glass strip via a knobby bottom roller during the hot shaping.
This knobby structure is composed of regular patterns of spherical caps or knobs that are round or oval or can also be of another shape. The knobs bring about a protection of the underside of the glass-ceramic plate against strength-reducing lesions.
The strength is lastly obtained in that the “lesions” of the underside are collected on the knob caps, so that in “valleys”, where the maximum dangerous tensile stresses occur under load, the notching effect is reduced, since the glass-ceramic surface has not been damaged.
The disadvantage of these knobs is the scattering of the light that is conducted through the glass-ceramic plate. It is not possible to make visible without distortion the displays or structures underneath the glass-ceramic plate. Displays and also the cooking zones are thus perceived in a slightly distorted manner.
The local introduction of a silicone layer in order to make visible without distortion the usual light-emitting displays is known from DE 41 04 983 C1. This silicone layer, however, introduces an additional expense, has poor transmittance behavior, and is less temperature-stable at the high heating temperatures of the cooktop. For this reason, this immersion layer can just be used locally in cold regions of the cooktop. The distorted view of the heating zones still exists.